1. Field of the Invention
Silicon carbide's unique combination of properties make it a particularly suitable material for a variety of applications in the semiconductor, optical, electronic and chemical processing fields. Moreover, chemical vapor deposition (CVD) techniques have been widely used to provide thin films and coatings of a variety of materials on various articles. Silicon carbide articles produced by chemical vapor deposition (CVD) processing are recognized to exhibit superior mechanical, thermal, physical and optical properties. This invention is directed to improvements in a CVD process of producing free standing, self-supporting silicon carbide articles, and is particularly adapted to the production of hollow shells of cylindrical, frustoconical or other shapes. Such shells can be used in x-ray telescopes, semiconductor processing furnaces, heat exchangers, laser tubes and chemical process equipment.
2. Description of Related Art
The advantages of silicon carbide as a fabrication material for astronomical X-ray telescopes and the experimental use of small scale CVD processing to prepare conical silicon carbide shells was recently described by Geril et al. in "Thin Shell Replication of Grazing Incident (Wolter Type I) SiC Mirrors", SPIE Proc., 2478, 215 (1995).
The advantages of CVD produced free-standing silicon carbide materials in applications requiring a high degree of surface smoothness and polishability are described in U.S. Pat. No. 5,374,412. The patent describes apparatus and process conditions which are used in the CVD production of free-standing silicon carbide articles. This patent also refers to earlier U.S. Pat. Nos. 4,990,374; 4,997,678 and 5,071,596 as further describing CVD processes of producing free-standing silicon carbide materials by the pyrolytic deposit of SiC on a mandrel.
Several methods of controlling or isolating the deposit of silicon carbide to the intended side of the substrate during chemical vapor deposition are described in U.S. Pat. Nos. 4,963,393 and 4,990,374. In U.S. Pat. No. 4,963,393, a curtain of a flexible graphite cloth is arranged to shield the backside of the substrate from the flowing reacted precursor gases, whereby silicon carbide deposits on the backside of the substrate are avoided. In U.S. Pat. No. 4,990,374 a counterflow of a non-reactive gas is directed to flow past the substrate's peripheral edge from behind the substrate whereby the deposit is confined to the front face of the substrate.